The present invention relates in general to condition control systems and, more particularly, to the changing and setting of analog voltage command signals, in response to change signals of variable duration, within such systems.
In many and diverse types of control systems, the condition to be controlled is affirmatively restored to a desired set point, whenever uncontrolled variables make it depart, by actuators which adjust final control elements (e.g., dampers, valves or heaters) to correctively influence the controlled condition. Each actuator is associated with or forms a part of an open or closed loop servo device which, in one way or another, must be told--by command signal intelligence--where its associated element should be positioned.
There are several classes of servo devices. Perhaps the most common class (here called Class I) are electrical and respond to the magnitude of a steady but settable dc. voltage or current to make the associated final element move to a corresponding position. Commonly, the required magnitude is determined, in response to sensed parameters, in a digital computer which is programmed to operate according to a predetermined algorithm. But the computer first arrives at and signals the required command magnitude numerically in digital form as a multi-bit binary word --and thus an associated digital-to-analog converter is required at a multi-bit output port of the computer for each servo device or "channel".
Another class of servo devices (here called Class II) responds to on-off "pulse" signals to change the position of the controlled element, with the amount of change being proportional to the time interval or duration during which a signal exists. For example, a pneumatic piston biased by a return spring is disposed in an air cylinder and connected to adjust a damper or valve (the final element). Air from a pressure source is admitted to the cylinder when a first solenoid valve is opened, but is bled to atmosphere when a second solenoid valve is opened. Each valve may be energized when an on-off signal from a corresponding single-bit computer output port exists. In this case, the computer algorithm is arranged to make the signal at one or the other of those ports "high" for a measured time interval when the element is to be moved in one direction or the other, the extent of movement being generally proportional to the time interval duration. The computer outputs a "high" signal at a given single-bit port for a computed time duration by following programming steps which constitute a timer loop.
In some prior arrangements of this sort, the servo device responds to a positive or negative polarity of a single on-off change signal to move the associated element in one direction or the other; the computer makes a polarity-selection port low or high during different alternating time spans; and the computer makes another single-bit output port high for a determined time duration within either of those spans. By relay logic or the like, the time duration signal is made of positive polarity when the first port signal is low but the second port signal is high. Conversely, the time duration signal is made of negative polarity when both such port signals are high. This permits the single-bit port for polarity selection to be shared by several "channels" respectively served by single-bit ports whose outputs are turned on for differing time durations when Class II servo devices are to be activated so that their actuators move their final elements by determined amounts and in desired directions.
It is often the case that the number and classes of servo devices required in a given control system is not known prior to the start of field installation. It may turn out that an insufficient quantity of one class of servo devices is on hand; and it may turn out that the computer at hand has an insufficient number of multi-bit output ports to accommodate the required quantity of Class I servo devices, recognizing that each of the latter requires a plurality of single-bit ports (say, eight) to output a binary digital number for each such device. And certainly, the digital-to-analog converter (DAC) normally required for each Class I servo device represents a considerable cost which is desirably to be avoided.